RNA viruses do not exist in nature as a single genomic sequence, but rather as an amalgam of related sequences referred to as a viral swarm or quasispecies. This proposal is designed to characterize the functional consequences of virus sequence variation within hosts that vary in immunological competence using WNV as a model. This proposal aims to identify how the type I IFN response differentially restricts viral growth an disease caused by a genetically homogenous versus heterogeneous virus population. It also addresses the effect of type I IFN on the evolution of the WNV genome, leading to a better understanding of the viral sequences and diversity (both nucleotide and protein) that are restricted by the type I IFN signaling pathway. A greater understanding of the interactions between type I IFN and a viral swarm could promote novel strategies to limit viral diversity, immune escape, and disease severity. In addition, understanding how type I IFN controls viral evolution could promote increased safety and immunogenicity of live attenuated vaccines, as most vaccines are derived from infectious clones, a source of homogenous virus.